Recently, color image forming apparatuses adopting electrophotography, inkjet printing, and the like require higher resolution and higher image quality. In particular, the tonality of a formed color image and the stability of density in a formed image greatly influence the image forming characteristics of the color image forming apparatus. It is known that the density of an image formed by the color image forming apparatus varies upon a change in environment and long-time use. Especially an electrophotographic color image forming apparatus loses the color balance of a formed image upon even small density variations, and efforts must be made to always keep its density characteristics to tonality constant. For this purpose, the electrophotographic color image forming apparatus obtains a stable image by the following density control process. First, a toner image (to be referred to as a patch hereinafter) for detecting density is formed on an intermediate transfer material, photosensitive member, or the like with toner of each color. Then, the density of the unfixed toner patch is detected by a toner density detection sensor (to be referred to as a density sensor hereinafter), the detection result is fed back to process conditions such as the amount of exposure light and the bias voltage for development.
In density control using the density sensor, a patch is formed on an intermediate transfer material, photosensitive drum, or the like, and detected. This density control cannot follow a change in the color balance of an image that is caused by variations in transfer and fixing characteristics onto a transfer material (sheet). To solve this problem, there has been proposed a color image forming apparatus which adopts a sensor (to be referred to as a color sensor hereinafter) for detecting the density or color of a patch transferred onto a transfer material (sheet) (Japanese Patent Laid-Open No. 2003-287934).
This color sensor can read a color patch on the transfer material and obtain an RGB signal corresponding to the color of the color patch. By performing density control (tonality control) using an output from the color sensor, higher-precision density control can be realized.
However, control using the color sensor consumes transfer materials and toner because a patch must be formed on a transfer material such as a recording paper sheet. Hence, density control using a patch cannot be frequently executed, and effective density control must be performed while minimizing the execution count of density or chromaticity control (to be referred to as color sensor control hereinafter) using the color sensor.
Density variations (also including variations in transfer/fixing characteristics) in an electrophotographic image forming apparatus occur depending on the state of the use environment, the conditions of an image pattern to be printed, and the like. The degree of density variations greatly changes depending on the use conditions of the apparatus, and is hard to predict. The density may or may not vary greatly depending on the difference in use conditions. Since the image forming apparatus must always form an image at a stable density, dolor sensor control must be executed assuming the case where the density varies most greatly (abruptly). That is, if color sensor control is done while the density of a formed image varies little, unnecessary control is executed, wastefully consuming toner or paper.